1. Field of the Invention
In an automatic transmission comprising a fluid type torque converter having an input member including a pump vane wheel and an output member including a turbine wheel, and an auxiliary transmission having one or more stages of gear trains, through which the torque of the output member is transmitted to a driving wheel, a lock-up control device for a torque converter in which a direct coupling clutch capable of mechanically coupling said input and output members is interposed therebetween to control the slip loss of the torque converter to the minimum.
2. Description of the Prior Art
When an accelerator pedal is released during the operation of the vehicle with the torque converter placed in a lock-up state by actuation of the direct coupling clutch, a sudden change in reaction applied to a power plant including the engine is transmitted to the vehicle body through the power plant and as a consequence, unpleasant surging tends to occur. It is effective to release the actuation of the direct coupling clutch at an idle position of an engine throttle valve in order to avoid the unpleasant surging. Various devices for that purpose have already been proposed.
A further means for solving this problem has already been proposed, in which means, a unidirectional clutch is interposed in series with the direct coupling clutch between the input and output members of the torque converter, said unidirectional clutch being designed so that at the time of operation of the direct coupling clutch, only the output torque of the engine may be transmitted towards the output member. In the idle condition of the engine throttle valve, vibration noises and wear due to the slip of the unidirectional clutch somewhat occur. If prevention of such occurrence of the noises or the like is taken into consideration, it is desirable to release the direct coupling clutch.
On the other hand, the applicant has already proposed an arrangement wherein for the purpose of maintaining the power performance during the operation of the direct coupling clutch, the slip characterisitc is imparted to the direct coupling clutch and the engaging force of the direct coupling clutch is controlled in accordance with the change in vehicle speed whereby during the maximum power output operation of the engine in a low vehicle speed zone, the slip is purposely created in the direct coupling clutch to thereby allot the transmission of output torque of the engine to a mechanical transmIssion system including the direct coupling clutch and a fluid transmission system including a fluid of the torque converter, thus suitably restoring a torque amplifying function of the torque converter, and in other words, power division of one kind is effected.
If an attempt is made to simultaneously effect the lock-up controls of different kinds, two valves are required, one valve for releasing the direct coupling clutch in the idle state of the engine throttle valve and the other for controlling the coupling force of the direct coupling clutch in accordance with the change in vehicle speed. If these can be controlled efficiently by a single valve, the device is materially simplified, and a leak loss of pressure oil irom the valve is minimized, which is extremely advantageous in practical use.